SAN FRANCISCO—Thousands of people convicted of marijuana offenses in this city going back to 1975 will have their convictions dismissed or reduced, San Francisco’s district attorney announced Wednesday.

It marks one of the most aggressive moves to wipe away old convictions in the face of new laws legalizing marijuana in California and other states.

San Francisco District Attorney
George Gascón
said that his office would dismiss and seal 3,038 misdemeanor marijuana convictions, and review and possibly resentence 4,940 felonies—all of which were adjudicated before California voters legalized marijuana in 2016.

Under the state legalization measure, Californians can petition the courts to get old marijuana possession and other convictions dismissed. Mr. Gascón said his office is taking the extra step of doing it for people in order to lift the burden of past convictions that can make it difficult for people to get jobs.

“A criminal conviction can be a barrier to employment, housing and other benefits, so instead of waiting for the community to take action, we’re taking action for the community,” said Mr. Gascón

Nine states and Washington, D.C., have legalized recreational marijuana use, and a debate has arisen over what to do with past pot convictions in these states.

In Nevada, where recreational marijuana was legalized,
Gov. Brian Sandoval
vetoed a bill last year that would have required certain offenses to be dismissed and sealed. Mr. Sandoval, a Republican, said in his veto message that such issues were better handled on a case-by-case basis.

In Colorado, prosecutors have raised concerns over bills making wiping away old pot convictions easier, said Arnold Hanuman of the Colorado District Attorneys’ Council.

“Many times convictions are plea bargained down from more serious conduct,” said Mr. Hanuman. “Our concern is that the original conduct involved in the incident is oftentimes more egregious.”

More states are including provisions in legalization measures for expunging past convictions, said Chris Lindsey, senior legislative counsel at the Marijuana Policy Project, which advocates for ending pot prohibitions. “This move by San Francisco is remarkable,” said Mr. Lindsey. “It’s not only do we allow people to repair their criminal histories, the local jurisdiction is just going to do it for them.”